1. Technical field of the Invention
The present invention relates to an apparatus and method for ELID grinding a large-diameter workpiece such as a silicon wafer to produce a mirror-like finish by using the ELID (electrolytic in-dressing) grinding method.
2. Prior Art
To cut a thin silicon wafer from an ingot of single crystal silicon, conventionally, tools such as (1) an outer-edge cutter, (2) an inner-edge cutter or (3) a wire saw are used.
A typical outer-edge cutter is shown in FIG. 1A; a thin disk-shaped cutting edge 2 with a center shaft 2a is rotated at a high speed, and its outer periphery cuts an ingot 1. The inner-edge cutter, shown in FIG. 1B, cuts an ingot 1 using an electrolytically deposited grindstone provided on the inner periphery of a thin disk-shaped cutter 3 with a center hole 3a by rotating the cutting disk at a high speed. The wire saw is shown in FIG. 1C, in which an endless fine wire 4 with a diameter of 0.2 to 0.3 mm is moved using a guide pulley 4a, while a slurry containing grinding grains is supplied between an ingot 1 and the wire 4, and cuts the ingot 1.
The aforementioned cutting methods require the following conditions to be fulfilled.
(1) Warping of the cut surface (and the entire wafer) must be kept small.
(2) The irregularity of the thickness of a cut wafer, that is TTV (Total Thickness Variation) should be small.
(3) The surface roughness should be small.
Next, the surface of the cut silicon wafer is, as shown generally in FIGS. 2A and 2B, ground by an infeed-type single-surface grinding device that grinds the surface thereof with a cup grindstone 6 while the workpiece 5 (silicon wafer) is rotated about the axis thereof. In addition, a dual-surface grinding device shown in FIG. 2C is also used in practice, in which the driving rollers 7 rotate the workpiece 5 about its own axis, and both surfaces of the workpiece are ground simultaneously.
Recently, silicon ingots have become large in diameter; although conventionally the diameter was limited to about 8 inches (about 200 mm), ingots of 12 inches (about 300 mm) and 16 inches (about 400 mm) can now be manufactured. However, when an ingot with such a large diameter is cut, the results are unavoidably poorer than for a small diameter ingot, in terms of warping of the cut surface, thickness variations and surface roughness.
Therefore, when an attempt is made to cut the surface of a large-diameter ingot as described above using the above-mentioned conventional infeed-type surface grinding equipment, the following problems are encountered.
(1) Because the diameter of the cup grindstone 6 must be larger than the diameter of the workpiece 5, the diameter of the cup grindstone 6 can be as large as, for instance, 500 mm. As a result, the cup grindstone can easily be deflected by the force required to press the cup grindstone onto the workpiece, so the accuracy of processing the surface of the workpiece becomes worse.
(2) Because the cup grindstone 6 must be large, the supporting mechanism and the driving device which rotates it also become large machines weighing as much as several ten of tons, therefore, the equipment is very expensive.
(3) As the cup grindstone becomes large, it takes a very long time to true it, and when electrolytic dressing is applied, large capacity electrolytic equipment is needed (for instance, several thousand amps or more), therefore, the power supply equipment must also be large and costly.
(4) When the workpiece is to be lapped in the next stage of the process after cutting, although it is preferable that the surface of the workpiece should be shaped with a slight recess instead of completely flat, conventional equipment can only cut a workpiece into a flat shape, or even if it can be done in principle, the equipment would become very complicated and the shape of the cut could not be controlled well.
The present invention is aimed at solving the problems described above. More explicitly, an object of the present invention is to provide an apparatus and method of ELID grinding a large-diameter workpiece to produce a mirror surface finish, that is capable of grinding the surface of a large-diameter wafer without using a large grindstone, that can maintain a high processing accuracy and a small deflection of the grindstone, that can be trued quickly and easily, that can be dressed electrolytically using a small power supply, and that can process the workpiece so that the shape of the cross section is other than a flat plane (for instance, a recessed shape).
According to the present invention, an apparatus is provided for ELID grinding a large-diameter workpiece to produce a mirror surface finish; the apparatus is comprised of a workpiece driving device (12) that makes the workpiece (5) rotate in horizontal, a cylindrical conducting grindstone (14) with an outer periphery that can contact the surface of the workpiece, a grindstone rotating device (16) that causes the aforementioned grindstone to rotate about the axis thereof, a grindstone reciprocating device (18) that drives the above-mentioned grindstone with a reciprocating motion along the line extending from the axis thereof, an axial guiding device (20) that maintain the center line of the axis of the above-mentioned grindstone at a predetermined angle to the horizontal axis, and an ELID device (22) that electrolytically dresses the outer periphery of the grindstone.
In addition, the present invention offers an apparatus for ELID grinding a large-diameter workpiece to a mirror surface finish; the apparatus is comprised of a workpiece driving device (12) that makes the workpiece (5) rotate in horizontal, a cylindrical conducting grindstone (14) with an outer periphery that can contact the surface of the workpiece, a grindstone rotating device (16) that causes the aforementioned grindstone to rotate about the axis thereof, a grindstone reciprocating device (18) that drives the above-mentioned grindstone with a reciprocating motion in a direction orthogonal to the axis of rotation of the workpiece, an axial guiding device (20) that maintains the center line of the axis of the above-mentioned grindstone at a predetermined angle to the horizontal axis, and the ELID device (22) that electrolytically dresses the outer periphery of the grindstone.
Furthermore, the present invention provides a method of ELID grinding a large-diameter workpiece to produce a mirror surface finish, wherein the flat workpiece (5) is driven to rotate in horizontal, the cylindrical conducting grindstone (14) the outer periphery of which is in contact with the surface of the workpiece and is driven so that it rotates in a horizontal direction about the axis thereof, the aforementioned grindstone is driven with a reciprocating motion along the surface of the workpiece, the center line of the above-mentioned grindstone is maintained at a predetermined angle with respect to the horizontal axis, and at the same time, the outer periphery of the grindstone is dressed electrolytically.
According to the apparatus and the method of the present invention, because the flat workpiece (5) is driven so that it rotates in horizontal by the workpiece driving device (12), and the outer periphery of the cylindrical conducting grindstone (14) the axis of which is substantially horizontal, grinds the surface of the workpiece as the outer periphery contacts the workpiece surface, the diameter of the conducting grindstone can be freely set regardless of the size of the workpiece. Consequently, even when the surface of a wafer with a diameter of, for instance, 16 inches (about 400 mm), is being ground the outer diameter of the grindstone can be made small, so the surface of a large-diameter wafer can be ground without needing a large grindstone.
Moreover, since the diameter of the conducting grindstone can be made small compared to that of the wafer and the bearing of the grindstone rotating device (16) can support the grindstone so that the grindstone can rotate about a substantially horizontal axis, the runout that may occur due to the reaction force of the grinding during the processing of the workpiece can be greatly reduced. In addition, because the ELID device (22) electrolytically dresses the outer periphery of the grindstone, the grinding reaction forces and the associated deflection can be reduced further, thereby the accuracy of processing the surface can be increased.
In addition, as the cylindrical conducting grindstone (14) can be small, both the truing and electrical dressing times are short, so the current needed for electrolytic dressing is small and small and low-cost power supply equipment can be used. Because the axis of the grindstone is kept at a predetermined angle xcex8 to the horizontal by means of the axial guiding device (20) and the grindstone is driven with a reciprocating motion along the axis by the grindstone reciprocating device (18), a wafer can be processed to give cross sections other than a flat surface (for example, a recess) by setting the angle xcex8 appropriately.
According to a preferred embodiment of the present invention, the workpiece driving device (12) is comprised of a plurality of driving rollers (13) that contact the outer periphery of the flat workpiece (5) and cause the workpiece to rotate; whereby a pair of upper and lower conducting grindstones (14) can contact the upper and lower surfaces of the workpiece, respectively.
Using this configuration, a plurality of driving rollers (13) drive the flat workpiece (5) so that it rotates, and a pair of upper and lower conducting grindstones (14) grind both surfaces of the workpiece at the same vertical position at the same time, and even if there are variations in thickness or warping of the workpiece, the workpiece can be ground and processed to a predetermined shape (flat, convex or concave).
The aforementioned conducting grindstone (14) is a metal-bonded grindstone. The ELID device (22) is provided with an ELID electrode (23) positioned close to but separate from the outer periphery of the grindstone, a voltage applying means (24) that applies DC voltage pulses between the grindstone as the positive electrode and the electrode as the negative electrode, and a processing fluid feeding means (25) that feeds a conducting processing fluid between the grindstone and the electrode; the workpiece is ground by the grindstone and, at the same time the outer periphery of the grindstone is dressed electrolytically.
In this configuration, since the surface of the metal-bonded grindstone can be sharpened precisely by electrolytic dressing, a cut surface can be finished to an excellent, flat surface very close to a mirror surface finish by using microscopic grinding grains. In addition, the load on the next process (polishing) can be greatly reduced, and processing damage to the crystals can be minimized.
Other objects and advantages of the present invention are revealed in the following paragraphs referring to the attached drawings.